Coating for ceramic composites

ABSTRACT

A coating composition for use with ceramic composites to reduce gas permeability of the composites as well as provide an adhesive force to the composites. The coating composition comprises an aqueous dispersion of an aluminum phosphate precursor, silicon carbide, and aluminoborosilicate.

This is a division of Application No. 07/684,317 filed Apr. 12, 1991,now U.S. Pat. No. 5,260,125.

FIELD OF THE INVENTION

This invention relates to a ceramic coating for high-temperature siliconcarbide ceramic composites used in gas-fired radiant burner tubes, gasburner nozzle liners, heat exchangers, and other furnace components. Theceramic coating of the present invention substantially matches thethermal expansion coefficient of the composites, thus resulting in agas-impermeable composite that still maintains its toughness. Theinvention also relates to using the ceramic coating to bond ceramiccomposites together.

BACKGROUND OF THE INVENTION

Furnace components such as radiant burner tubes must be able towithstand high temperatures and corrosive environments in industrialheat-treating and in aluminum melting furnaces. Commercially-availableburner tubes operate in the range from about 900° C. to about 1250° C.and are generally metal alloy tubes, ceramic monolith tubes, and ceramiccomposite tubes. Of the first type, nickel-based superalloy metal tubesare commonly used, but are limited to the lower temperature range of900°-1100° C. Of the second type, monolithic silicon carbide radiantburner tubes are commonly used and generally have temperaturecapabilities up to about 1250° C. but suffer from the brittle failureproblems typical of monolithic ceramic shapes. Furnace components, usedin very high temperatures and in corrosive environments, require aspecial selection of materials to avoid chemical and mechanicaldisintegration of the ceramic. Ceramic-ceramic composites, using ceramicfibers and cloths as reinforcements in a ceramic matrix, are the thirdtype of tube and are frequently the most desirable choice for use inhigh temperature, chemically-corrosive environments.

One type of commercially-available radiant burner tube is produced underthe designation SICONEX(TM) Fiber-Reinforced Ceramic, and iscommercially available from Minnesota Mining and Manufacturing Company,St. Paul, Minn. SICONEX(TM) Fiber-Reinforced Ceramic is aceramic-ceramic composite comprising aluminoborosilicate fibers in asilicon carbide matrix. SICONEX(TM) Fiber-Reinforced Ceramic is preparedby first forming a tube or other shape of NEXTEL(TM) aluminoborosilicateceramic fibers (commercially available from Minnesota Mining andManufacturing Company, St. Paul, Minn.) by braiding, weaving, orfilament-winding the ceramic fibers. The ceramic fiber shape is treatedwith a phenolic resin to rigidize it, and then coated via chemical vapordeposition at temperatures ranging from 900° to 1200° C. to produce arelatively impermeable, chemically-resistant matrix of a refractorymaterial such as beta-silicon carbide. The resultant rigid ceramiccomposite is then useful at high temperatures and in corrosiveenvironments.

However, the utility of these materials as furnace components can,depending on the degree of their permeability to gases, be somewhatlimited. Ceramic-ceramic composites such as SICONEX(TM) are comprised ofrelatively open networks of fibers and can remain permeable to gases,even after extensive overcoating with a ceramic (e.g., silicon carbide)layer.

While there have been many approaches to sealing ceramic compositesurfaces, these attempts have not been coupled with sufficient matchingof chemical, thermal, and mechanical properties of the coating toachieve adequate thermal and chemical behavior at extreme temperaturesand reaction conditions. Thermal expansion coefficient matching isespecially critical due to the elevated temperatures of use and repeatedthermal cycling in typical furnace applications.

Previous work in this field generally is directed to coating, sealing,or adhering refractory materials. U.S. Pat. No. 4,358,500 and relatedU.S. Pat. No. 4,563,219 describe a composition for bonding refractorymaterials to a porous base fabric such as fiberglass, using a coatingcomprised of colloidal silica, monoaluminum phosphate, and aluminumchlorohydrate. The coating provides heat and flame protection to thefiberglass fabric.

U.S. Pat. No. 4,507,355 describes an inorganic binder prepared fromcolloidal silica, monoaluminum phosphate, aluminum chlorohydrate and acatalyst of alkyl tin halide. This mixture is applied to the preferredsubstrate fiberglass to form a heat-resistant fabric.

U.S. Pat. No. 4,592,966 teaches a method of strengthening a substrate(fiberglass or fiberglass composites) by impregnating the substratewith, for example, aluminum or magnesium phosphate, magnesium oxide, orwollastonite, and a non-reactive phosphate. This is described as acement which lends strength to the fiber substrate.

U.S. Pat. No. 4,650,775 describes a thermally-bonded fibrous productwherein aluminosilicate fibers are bonded together with silica powderand boron nitride powder. These mixtures can be formed into differentshapes and used as diesel soot filters, kiln furniture, combustorliners, and burner tubes.

U.S. Pat. No. 4,711,666 and related U.S. Pat. No. 4,769,074 describe anoxidation prevention coating for graphite. A binder/suspension ofcolloidal silica, mono-aluminum phosphate and ethyl alcohol is appliedto a graphite surface and prevents oxidation during heat cycling.

U.S. Pat. No. 4,861,410 describes a method of joining a metal oxideceramic body such as alumina with a paste of a sol of a metal oxide,aluminum nitrate, and silicon carbide. This method is used to repaircracks in ceramic materials and to permanently join ceramic structurestogether.

Silicon carbide-ceramic fiber composites would benefit greatly from acoating that would protect the composites in high temperature andcorrosive environments. To be most effective for high temperature uses,the coating needs to match the thermal expansion coefficient of thecomposite. In uses which require minimal transfer of gases through thewalls, the coating needs to reduce the permeability of the siliconcarbide-ceramic fiber composite. A further need in this field is theability to adjoin ceramic composite pieces together or to patch holes inthe composite articles.

To date, there has not been a coating composition which matches thethermal expansion coefficient of an aluminoborosilicate fiber-siliconcarbide-coated composite under high temperature conditions, limits gaspermeability and can be used to adjoin the aforementioned compositestogether. The increased use of ceramic composites in high temperatureand corrosive environments creates a need for a coating composition withthe above attributes.

SUMMARY OF THE INVENTION

An impermeable, ceramic-ceramic composite is formed by coating a siliconcarbide coated composite of aluminoborosilicate fibers with a ceramicprecursor coating comprised of an aqueous suspension of an aluminumphosphate precursor, flakes or chopped fibers of aluminoborosilicate andsilicon carbide powder, flakes, or fibers. The term "impermeable" ismeant to denote a coating which is substantially impermeable to gasespassing through the coating. The coating can be applied by spraying,dipping, or brushing. The coating is dried in air and then fired to forma hard and durable coating.

By application of this coating, the strength of the ceramic composite,as measured by internal pressurization to failure, is equal to orslightly higher than that of an uncoated composite tube. This behavioris an important indicator of the composite character of the final coatedstructure. It is particularly desirable to avoid firing or reactingcomposite materials to a point at which the composite actually takes onthe characteristics of a monolithic structure. In a practical sense, theresult of monolithic behavior is a dramatically increased brittleness ofthe material; hence, monolithic structures are dramatically lesseffective for uses which subject the material to mechanical stress. Aceramic composite having the coating of the present invention results ina tough structure and not a monolithic structure. The coating may alsobe used as a bond coating which secures two ceramic substrates,particularly tubes, together.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with this invention, silicon carbide ceramic compositesare coated with an aqueous suspension of monoaluminum phosphate (Al(H₂PO₄)₃, flakes or chopped fibers of aluminoborosilicate, and siliconcarbide powder. The coating is most easily applied by brushing it ontothe composite surface, although other application methods, such as dipcoating or spraying, could be used. Once the coating is applied to thecomposite, it is allowed to air dry, and then fired to about 1000° C. toform a hard and durable ceramic coating.

There are many silicon carbide ceramic composites which could be used inconjunction with the coating compositions of the present invention. Onebrand of composite is the afore-mentioned SICONEX(TM) Fiber-ReinforcedCeramic, commercially-available from Minnesota Mining and ManufacturingCompany, St. Paul, Minn. These composites are formed by first braiding,weaving, or filament-winding fibers of aluminoborosilicate (sold underthe trade designation NEXTEL(TM) 312, having an alumina:boria mole ratioof from 9:2 to 3:1.5 and containing up to 65 weight percent silica, asdescribed in U.S. Pat. No. 3,795,524, assigned to Minnesota Mining andManufacturing Company) to form a desired shape, such as a tube. The tubeis coated with a phenolic resin in an organic solvent to rigidize it andthereafter coated with silicon carbide via chemical vapor deposition.

The coating of the present invention is comprised of silicon carbide,aluminum phosphate and aluminoborosilicate. An available source ofsilicon carbide is available as fine abrasive powder, commerciallyavailable from FuJimi Kenzamaki Kogyo Co., Inc., Nagoya, Japan. Otherforms of silicon carbide include flakes or fibers. In the preferredembodiment, 1-50 micrometer diameter silicon carbide powder is used.

The precursor aluminum phosphate present in the coating is prepared bydissolving aluminum metal in phosphoric acid. A solution, 50 weightpercent of Al(H₂ PO₄)₃ in water, is available from Stouffer ChemicalCompany, Westport, Conn. As the coating is fired, water and a portion ofphosphate are released from the aluminum phosphate solution. Thus,aluminum phosphate is left after firing. The mole ratio of siliconcarbide to aluminum phosphate (SiC:AlPO₄) in the fired coating ispreferably in the range of about 1:1 to 50:1. Most preferably, the moleratio of SiC:AlPO₄ in a fired coating is in the range of about 5:1 to30:1.

Aluminoborosilicate is also added to the coating composition. This maybe in the form of powder, flakes or fibers. Preferably,aluminoborosilicate, in the form of fibers, is used and is availableunder the trade designation NEXTEL(TM) Ceramic Fiber, commerciallyavailable from Minnesota Mining and Manufacturing Company. The ceramicfiber yarn ranges in diameter from 11 to 15 micrometers and is choppedby passing the yarn between two steel rollers with knurled surfaces.Other methods of chopping include ball milling or other methods known inthe art. The yarn is chopped to an average fiber length of about 0.02 to0.05 mm. The weight percent of the aluminoborosilicate of the totalfired coating composition is in the range of about 0.5 to 70% and,preferably, in the range of about 30 to 70%.

To fashion the ceramic composites for testing the different coatingcompositions of the present invention, NEXTEL(TM) Ceramic Fiber braidwas fit onto a 5 cm diameter aluminum mandrel, and a solution of 10 mlof phenolic resin (DURITE(TM) SC-1008 phenolic resin, 60-64% solids,commercially available from Borden Chemical, Columbus, Ohio) in 90 ml ofmethanol was prepared. A small amount of the resin solution was pouredover the ceramic fiber tube while rotating the mandrel, to assureuniform coverage by the resin. The tube was then dried in air untilsolvent odor could no longer be detected, and then cured in air at 200°C. for 20 minutes. This process resulted in a rigid tube having a goldencolor due to the cured polymer layer.

The rigid preform was placed in a chemical vapor deposition chamber, asis well known in the art, wherein vacuum is applied and the chamber isheated. Hydrogen gas was bubbled through dimethyldichlorosilane (DDS)and passed through the CVD furnace chamber, thermally decomposing theDDS which thereby deposited a layer of silicon carbide on the preform.By-product and unreacted gases exited the opposite end of the tube tothe vacuum pumping and scrubbing system. Typical process conditions forthese samples were pressures of 5 to 50 tort, flow rates of 0.15 litersper minute(lpm) of DDS, and 1.0 lpm of hydrogen gas at temperatures of900° to 1000° C. Coating times ranged from 4 to 8 hours. Under theseprocess conditions and times, the samples received from about 100 toabout 200 weight percent increase due to silicon carbide deposition. Inthis process, SiC coats and infiltrates the fibers and the resin coat isalso decomposed to form a carbonaceous layer on the surface of thepreform. It is useful to examine the fractured surfaces of brokencomposites made in the above manner. The fractured surfaces resulted ina "brushy" fracture surface which indicates that the coated material hascomposite rather than monolithic properties, and that heating andprocessing steps have not destroyed the desired composite properties.

Coupons of SICONEX(TM) Fiber-Reinforced Ceramic were prepared in amanner similar to the tubes, using woven ceramic fiber (NEXTEL(TM) 312)fabric. Coupons were convenient for carrying out initial studies ofcoating feasibility and were more convenient to use in order to examinethe adhesion and hardness of the coating. Adhesion of the coating on anexposed edge and the performance of the coated edge are also importantindicators of the coating performance.

Many sizes of tubes of the ceramic-ceramic composite were coated andtested. Permeability of the final fired tubes was tested by adifferential flow test using a flow meter.

Though not being bound by theory, it is believed that the coating worksto maintain the composite characteristics of its composite substrate aswell as to match the thermal expansion coefficient of the substrate(which is important in furnace and high temperature applications)because the coating itself is a composite material, being comprised offlakes or fibers and particles in a matrix. The flakes, fibers, andparticles act to fill the porous sites in the matrix, thereby blockingthe flow of gas through the porous sites. Further, this discontinuousphase also deflects cracks that may initiate in the coating frommechanical or thermal stresses.

EXAMPLE 1

Aluminoborosilicate (NEXTEL(TM) 312 ceramic fiber commercially-availablefrom Minnnesota Mining and Manufacturing Company, St. Paul, Minnesota)ranging in diameter from 11 to 15 micrometers was chopped by passing theceramic fiber yarn between two steel rollers with knurled surfaces. Thisresulted in chopped fibers with an average length of about 50micrometers.

To a 50 percent by weight solution of monoaluminum phosphate, (Al(H₂PO₄)₃, commercially available from Stauffer, Westport, Conn.) was addedsilicon carbide powder (#1500, 8micron, commercially available fromFujimi Kenmazai Kogyo Co., Ltd. Nagoya, Japan) and chopped NEXTEL(TM)312 ceramic fiber. Deionized water was added to some mixtures to adjustthe consistency for coatability. Table I shows compositions representingapproximately 40-70% fired solids and mole ratios of SiC to AlPO₄ in thefired product of from about 5 to about 20:1.

                  TABLE I                                                         ______________________________________                                        COATING COMPOSITIONS                                                          component    mass, g moles SiC:AlPO.sub.4                                                                       % fired solids                              ______________________________________                                        a.  Al(H.sub.2 PO.sub.4).sub.3                                                                 2.9     6          40                                            SiC powder   1.1                                                              NEXTEL(TM)   1.3                                                              fiber                                                                         deionized    2.0                                                              water                                                                     b.  Al(H.sub.2 PO.sub.4).sub.3                                                                 45.0    6          50                                            SiC powder   16.3                                                             NEXTEL(TM)   11.4                                                             fiber                                                                         deionized    --                                                               water                                                                     c.  Al(H.sub.2 PO.sub.4).sub.3                                                                 50.0    6          55                                            SiC powder   18.1                                                             NEXTEL(TM)   21.7                                                             fiber                                                                         deionized    --                                                               water                                                                     d.  Al(H.sub.2 PO.sub.4).sub.3                                                                 5.0     16         69                                            SiC powder   5.0                                                              NEXTEL(TM)   5.0                                                              fiber                                                                         deionized    1.0                                                              water                                                                     e.  Al(H.sub.2 PO.sub.4).sub.3                                                                 1.5     20         47                                            SiC powder   1.9                                                              NEXTEL(TM)   1.9                                                              fiber                                                                         deionized    4.0                                                              water                                                                     ______________________________________                                    

Tube-shaped SICONEX(TM) Fiber-Reinforced Ceramic samples were dipped in,or painted with, each coating formulation, typically in only one pass.Coated parts typically weighed 10 to 20% more than the weight of theoriginal part and had a coating thickness of about 1 mm. The coatedparts were allowed to dry at ambient temperature and humidity for 24hours and then were fired in air by ramping the temperature at 250° C.per hour to 1000° C., and holding for 1 hour. The coatings were hard anddurable as indicated by attempting to remove or crack the coating byscratching the surface with a steel needle. Intact ceramic fibers andparticles of SiC could be seen by examination under a microscope at 50Xmagnification. X-ray diffraction powder patterns of the fired coatingsshowed beta-SiC, mullite, and ALPO₄ as crystalline phases.

EXAMPLE 2

In order to test the permeability of a sample before and after coating,tube-shaped samples were used. Through-wall permeability of two tubes(5.0 cm outer diameter×20.0 cm long) was measured by closing each end ofthe tube with a one-hole stopper, and flowing air through the tube. Airat a regulated pressure of 1 atmosphere (1.03 Kg/cm²) was admittedthrough a needle valve and monitored by a flow meter at the inlet end ofthe tube. A manometer at the exit end of the tube measured thedifference in pressure between the inside of the tube (pressurized airflowing through it) and the outside of the tube (room pressure). For aparticular pressure drop, the air flow in cm³ /min is read from the flowmeter. This flow rate, divided by the surface area of the tube, ispermeability (cubic centimeters per minute per square centimeter).

A coating of 55 weight percent fired solids and a 6:1 SiC:AlPO₄ moleratio (as per Example 1c) was applied to the outside surface of thetubes. The wet coating was 12 to of the original part weight. After airdrying, the tubes were fired to 1000° C. The tubes were weighed andpermeability checked again. Table II shows weight and permeabilitychanges:

                  TABLE II                                                        ______________________________________                                        PERMEABILITY DATA                                                                                   permeability                                            weight (gm)               (cm.sup.3 min.sup.-1 cm.sup.-2)                                    coated                   coated                                tube  uncoated & fired  % wt. gain                                                                            uncoated                                                                              & fired                               ______________________________________                                        1     77.62    85.19    9.8%    132.0   1.2                                   2     95.80    103.86   8.6     10.2    <.02                                  ______________________________________                                         Gas permeability was reduced by a factor of approximately 100 for tube 1      and a factor of 500 for tube 2.                                          

EXAMPLE 3

Two 5.0×20.3 cm SICONEX(TM) Fiber-Reinforced Ceramic composite tubeswere coated as described in Example 1 with the coating formulation ofExample 1c (designated A in Table III, below), and two tubes with nocoating (designated B in Table III) were fired together to 1000° C. for1 hour. All tubes were cut into 2.5 cm long rings in order to dostrength testing.

Additional samples were prepared to evaluate the coating as an edgeprotector for SICONEX(TM) Fiber-Reinforced Ceramic. Three 15.2 cm (6")samples were cut from one 5.1 by 45.7 cm (2"×18") tube and treated asfollows: Sample C (ends of 15.2 cm piece coated, heat treated to 1250°C. for 10 hours), Sample D (cut into 1" samples, cut edge coated, heattreated at 1250° C. for 10 hours), and Sample E (cut into 1" samples,heat treated at 1250° C. for 10 hours).

Burst strength was measured on 1" rings from all tubes by internalpressurization to failure (burst test); average results of the samplesare shown in Table III.

                  TABLE III                                                       ______________________________________                                        STRENGTH DATA                                                                 treatment        burst strength average                                                                       st. dev.                                      ______________________________________                                        1000° C., 1 hr.                                                        A coated             9420 psi   540                                           B uncoated       9230           1470                                          1250° C., 10 hr.                                                       C HT. as piece, cut                                                                            8000           920                                           D cut, edge coated, HT.                                                                        6840           1080                                          E cut, no coating, HT.                                                                         5420           670                                           ______________________________________                                    

In comparing Samples A and B, the burst strength of the samples showssome improvement after coating.

In the data for Sample C (15 cm-long sample, heated, sectioned, andtested) and E (six 2.5 cm ring samples, heated, and tested), it appearedthat cutting samples before heat treating resulted in a loss of strengthof about 33% with uncut samples. Cut samples which were also edge-coated(Sample D) suffered only about a 15% strength loss. Fracture surfaces ofSamples C and D are "brushy" (meaning individual fibers are visible andhave not fused together during heat treatment) and composite-like, whilefractured samples of E were quite brittle with less evidence of fiberpull-out. Although not intending to be held to any theory, it isspeculated that unprotected edges allow oxygen to penetrate into theinterface between fibers and the matrix. Oxidation within the matrix issuspected to result in bonding between the fibers and the matrix and,thus, brittle fracture behavior results.

EXAMPLE 4

Three coating formulations were prepared as described in Example 1 withthe formulation of Example 1d, except that the particle size of the SiCwas varied. The particle sizes were one micron, 8 micron, and 50 micronSiC powders, commercially available from Fujimi^(!) Kenmazai Kogyo Co.Ltd., Nagoya, Japan. Small SICONEX(TM) Fiber-Reinforced Ceramiccomposite samples were painted with the coatings and fired first to1000° C. for a period of one hour at a heat-up rate of 250° C./hour andthen to 1200° C. for a period of one hour. Each sample was hard anddurable as indicated by visual inspection after attempting to remove orcrack the coating by scratching the surface with a steel needle. Thus, awide range of silicon carbide particle sizes and a wide firingtemperature range produce acceptable coatings.

EXAMPLE 5

This example shows how the coating compositions can be used as adhesivesto join two samples together. To test for shear strength of the coatingwhen used as a bonding agent, 2.5 cm-long SICONEX(TM) Fiber-ReinforcedCeramic tubes of two different diameters were used (5 cm and 4.4 cm inouter diameter).

The tubes were Joined together by fitting the smaller diameter tubepart-way into the larger tube, such that the smaller diameter tubeprojected 1.25 cm out of the larger diameter tube. A 1.25 cm band ofcoating (70 weight % solids) was placed on the outer surface of thesmaller tube, and then a 1.25 cm wide piece of NEXTEL(TM) 312 ceramicfiber tape was placed on the coating. Additional coating was added tothe tape, ana then the tube with the coating and the ceramic fiber tapewas fitted into the larger tube.

Additional coating was added to fill the gap between the two tubes. Thisbonded piece was dried for 24 hours at ambient temperature and humidity,heated for 10 hours at 110° C., and fired for 2 hours at 1000° C.

An axial compression test of the joined tubes was carried out. In thistest, pressure was applied to the long axis of the joined tubes to tryto break the adhesive bond formed by the dried and fired coating betweenthe two tubes. Axial compression tests of fired tubes were carried outat 0.051 cm/min (0.02"/min) crosshead speed with an Instron Model 1125load frame. Joints tested in this way did not fail under a 1000 lb. (455Kg) load at room temperature. This indicates that the coating can beused effectively to join SICONEX(TM) Fiber-Reinforced Ceramic compositetubes together. This is useful for making T- or U-shaped tubes, or forcases in which the tube diameter must change in order to fit anotherpiece.

A further test of the bonding strength of the coating was to rapidlycycle joined pieces through a heating and cooling sequence. Two 5-cmlong by 4.4 cm diameter SICONEX(TM) Fiber-Reinforced Ceramic compositetubes were butt-joined using the coating composition prepared asdescribed above. An outer sleeve of 5 cm diameter and 2.5 cm long wasadded at the joint to further reinforce the butt-joint. The assembledtube was dried and fired as described above. The joined tubes wereflame-cycle tested by heating the inside of the joined tubes with thegas flame of a Meeker burner to a temperature of approximately 800° C.while cooling the outside of the tube with a flow of compressed air.These heat cycles did not cause failure of the bonds. Further heating ofthis heat-cycled joint for 100 hours at 1000° C. in air-caused nodetectable strength change.

EXAMPLE 6

In order to show utility of the coating formulations as an adhesive forpatching SICONEX(TM) composite parts together, a coating with 70 weight% solids was applied by brushing it onto a SICONEX(TM) composite tube,drying in air for several hours, and firing with a gas-air torch of thekind typically used for glass working. Components of the coating meltedslightly, lightened in color, and then hardened.

The coating is, thus, effective in attaching a patch to a SICONEX(TM)Fiber-Reinforced Ceramic composite tube with a hole in it or in bridgingsmall gaps or cracks in SICONEX(TM) Fiber-Reinforced Ceramic compositetubes in situations where the tubes are in need of repair and requirespot heat-treating.

As will be apparent to those skilled in the art, various othermodifications can be carried out for the above disclosure withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. An unfired coating composition useful forreducing the gas permeability of a silicon carbide coatedaluminoborosilicate ceramic composite, said unfired coating compositionconsisting essentially of:(a) an aluminum phosphate solution prepared bydissolving aluminum metal in phosphoric acid; (b) silicon carbide in anamount sufficient to create a fired molar ratio range of silicon carbideto aluminum phosphate of about 1:1 to 50:1; and (c) particles ofaluminoborosilicate dispersed therein in the weight range of about 0.5to 70 weight percent of the total weight of said unfired coatingcomposition when fired.
 2. An unfired coating composition useful forreducing the gas permeability of a silicon carbide coatedaluminoborosilicate ceramic composite, said unfired coating compositioncomprising:(a) an aluminum phosphate solution prepared by dissolvingaluminum metal in phosphoric acid; (b) silicon carbide in an amountsufficient to create a fired molar ratio range of silicon carbide toaluminum phosphate of about 1:1 to 50:1; and (c) fibers ofaluminoborosilicate dispersed therein in the weight range of about 0.5to 70 weight percent of the total weight of said unfired coatingcomposition when fired.
 3. The coating composition of claim 1 whereinsaid aluminoborosilicate particles are flakes.
 4. The coatingcomposition of claim 1 wherein said fired molar ratio of silicon carbideto aluminum phosphate is about 5:1 to 30:1.
 5. The coating compositionof claim 1 wherein said weight range of aluminoborosilicate particles isabout 30 to 70 weight percent of the total mixture.
 6. The coatingcomposition of claim 2 wherein said fired molar ratio range of siliconcarbide to aluminum phosphate is about 5:1 to 30:1.
 7. The coatingcomposition of claim 2 wherein said weight range of said fibers ofaluminoborosilicate is about 30 to 70 weight percent of the total weightof said unfired coating composition when fired.